The present invention relates to light passive display devices, such as liquid crystal displays, electrochromic displays, PLZTs, and the like. More particularly, the invention relates to improvements in the structure of such devices which provide improved visibility of the display, enhancing visual recognition, and to a method of producing the display device. Although the invention is described herein by way of example in the context of a liquid crystal display of the light-passive type, it is to be understood that the present invention is applicable to other displays.
A liquid crystal display which uses light-passive display elements depends, for its visual effect, upon external light and is therefore subject to degredation resulting from side effects fo the external light, such as reflection from the display surface. In this regard, such displays differ to a considerable degree from luminous display devices such as a CRT (cathode ray tube), an LED (a light-emitting diode display), etc., where the source of illumination is internal to the device. Thus, in light-passive display devices, incident light which is reflected by the first surface of the display device is not available for reflection therethrough and a further degredation of display performance results.
While the visibility of the display can be improved by covering the front surface of the liquid crystal display element with an interfering filter, such as a thin polarizing film on a plate, or by reducing the direct reflection of light from the front by altering the surface of the polarizing plate or of the device so as to scatter the light, such plastic surfaces are easily damaged and further reduction of the reflected light ensues due to the wear and tear. In one attempt to solve the problem, the effect of wear was minimized by using a replaceable plastic sheet; in another case, a glass plate was used which provides a surface which is less subject to wear. However, the use of the added protective sheet or plate results in degredation of visibility because of reflections produced at their surfaces. Some of the problems due to reflections can be solved by causing the plastic sheet or glass plate to adhere to the face of the liquid crystal display element. Heretofore, however, there has been no method of inexpensively causing such a transparent sheet or plate to adhere in a stable manner through the use of curing temperatures which would not affect the liquid crystal display element.
Conventionally, then, such methods have used a liquid adhesive agent, or the like, to produce adhesion. However, when a liquid adhesive agent is used, special equipment is required for the application of solvent, and, even when no solvent is required, handling of the liquid adhesive agent is tricky and difficult. On the other hand, when a tacky adhesive agent is used, dust and impurities are apt to be deposited thereon and so to be included in creases or folds in the surface which cannot easily be removed, and the result is poor adhesion.
The foregoing problems have been particularly acute in fabricating displays which require accurate adhesion of the protective plate so as to produce optical clarity. Further, when the above-mentioned methods are used, the process takes a great amount of time because of the number of substrates which much be caused to adhere in succession.